This invention relates to combustion engine driven vehicles and particularly to such vehicles adapted to operate on a mixture of fuels such as gasoline and alcohol in unknown, variable concentrations. The engines of such vehicles are optimally operated with one or more engine operating parameters varied in response to a fuel composition signal generated by a sensor in the engine fuel supply which is responsive to a parameter of the fuel mixture indicating fuel composition.
Several types of fuel composition sensors for vehicle engines are known in the prior art. These include optical, microwave and capacitive sensors. In the field of capacitive fuel composition sensors, a problem which must be overcome is the electrical conductivity of alcohols, which tends to greatly decrease accuracy of the sensors at alcohol concentrations greater than 30 percent, due to the short circuiting of the capacitor plates through the fuel mixture.
U.S. Pat. No. 4,915,084, Combustion Engine with Multi-Fuel Capability, issued Apr. 10, 1989 to Eugene V. Gonze and assigned to an assignee of this application, describes a capacitive fuel composition sensor capable of operating accurately at alcohol concentrations up to 100 percent. This sensor provides a capacitor in the engine fuel line, an electrical reference impedance in series combination with the capacitor, and an oscillator providing an oscillating voltage to the capacitor/impedance combination with a voltage component having a frequency of at least 1 Megahertz but less than a frequency requiring microwave circuit apparatus. The sensor further includes an electric circuit responsive to a capacitively determined electrical parameter of the capacitor to determine the capacitance thereof and therefore the dielectric constant of the fuel mixture. The high frequency of the voltage component used for capacitance sensing provides a capacitor impedance significantly lower than the parallel equivalent resistive impedance of the fuel due to fuel conductivity, so that the capacitive impedance predominates and the sensor thus provides an accurate indication of capacitance.